Plaguelands are regions where the Spellplague still runs amok, changing everything it contacts. Most plaguelands appear to be shrouded in a thick, luminous blue fog, with everything inside blurred and wavering when viewed from outside.

    Effect: The terrain inside the plaguelands boundary is mutable, shifting constantly like boiling mud. Each hour that a creature not already spellscarred or plaguechanged spends in a plagueland subjects it to the Spellplague.
    A creature that takes an extended rest in a plagueland takes a -1 penalty to Fortitude and loses one healing surge, which can only be regained outside the plagueland. This penalty stacks, but disappears when the creature takes an extended rest outside the plagueland. If a creature’s Fortitude drops to 0, it dies in an explosion of blue flame.
    Characters who survive a full day (24 or more consecutive hours) in a plagueland might qualify to choose a spellscar, at the Dungeon Master’s discretion.
    Usage: Due to the nature of the plaguelands, encounters inside them that include aberrations fit thematically, though almost any type of creature could be found in plaguelands. If the characters are in a plaguelands for a short time, use creatures with attacks targeting Fortitude to take advantage of that penalty. Be aware of the healing surge penalty if the characters will be in the plaguelands for an extended time, however, because the characters will have less resources to survive the longer they are there.

Published in Forgotten Realms Campaign Guide, page(s) 54.